1. Field of the Invention
The present invention relates to an apparatus for composing two screens in a television, and more particularly, to an improved apparatus for composing two screens in a television which is capable of solving the problems of a jitter and a screen being cut off at the top and bottom portions caused by a non reference signal in composing two screens.
2. Description of the Prior Art
Generally, as shown in FIG. 1, an apparatus for composing two screens on a television includes a first analog/digital (hereinafter, called A/D) converter 100 for converting a video signal for a main screen (VBS1) into a digital signal, a second A/D converter 110 for converting a video signal (VBS2) for a sub-screen into a digital signal, a line memory 120 for storing an output from the first A/D converter 100 in the unit of lines, a frame memory 130 for storing an output from the second A/D converter 110 in the unit of frames, a mixing unit 140 for receiving outputs from the line memory 120 and the frame memory 130 and mixing the outputs to compose two screens, a digital/analog (hereinafter, called D/A) converter 150 for converting the output from the mixing unit 140 into an analog signal, an encoder 160 for encoding an output from the D/A converter 150 for a display on the screen and outputting a double screen signal (DSS), a first clock signal oscillator 170 for receiving a horizontal synchronous signal (Sync1) for a main screen and generating a clock signal corresponding thereto, a second clock signal oscillator 180 for receiving an horizontal synchronous signal (Sync2) for a sub-screen and generating a clock signal corresponding thereto, and a clock/timing generator 190 for receiving outputs from the first and second clock signal oscillators 170,180 and generating a clock signal and a timing pulse signal required in each unit.
With reference to FIGS. 1 and 2, the operation and effect of the apparatus for composing two screens on a television having the above construction will now be described in detail.
First, the first A/D converter 100 converts the video signal (VBS1) for a main screen into a digital signal appropriate for processing, and the line memory 120 stores in the unit of lines the video signal (VBS1) for a main screen converted into a digital signal in accordance with a write enable signal shown in FIG. 2E-2. Here, the signal shown in FIG. 2E-2 is used as the write enable signal.
Here, as shown in FIG. 2E-1, a reset signal applied to the line memory 120 is generated synchronized with the horizontal synchronous signal (Sync1) for a main screen in FIG. 2A, after a predetermined time (a) passes, and as shown in FIG. 2E-2, a write enable signal is generated synchronized with the horizontal synchronous signal (Sync1) for a main screen in FIG. 2A, after a predetermined time (b) passes to store a set number of samples (c) in the line memory 120.
The second A/D converter 110 converts the video signal (VBS2) for a sub-screen into a digital signal appropriate for processing. The frame memory 130 stores in the unit of frames the video signal (VBS2) for a sub-screen converted into a digital signal in accordance with a write enable signal shown in FIG. 2F-2.
Then, a reset signal (a control signal) applied to the frame memory 130 is generated synchronized with the horizontal synchronous signal (Sync2) for a sub-screen shown in FIG. 2B, and the write enable signal shown in FIG. 2F-2 is synchronized with the horizontal synchronous signal (Sync2) for a sub-screen, and a predetermined number of samples (i) in the frame memory 130 is stored after a predetermined time elapses.
Since data is read in a frequency twice as large as a write clock signal such as a read enable signal illustrated in FIGS. 2E-3 and 2F-1 when the data is read from the line memory 120 and the frame memory 130, the image data outputted from the line memory 120 and the frame memory 130 is twice compressed. Here, when data is read from the line memory 120, as shown in FIG. 2E-3, a number of samples in (e) identical to those in (c) of FIG. 2E-2 are read out after a predetermined time (d) passes from the horizontal synchronous signal (Sync1) for a main picture, and when data is read out from the frame memory 130, as shown in FIG. 2F-1, a number of samples in (g) identical to those in (i) of FIG. 2F-2 are read out, after a predetermined time (d+e) passes from the horizontal synchronous signal (Sync1) for a main screen.
Therefore, the outputs from the line memory 120 and the frame memory 130 are mixed in the mixing unit 140 to be converted into a two-screen analog video signal through the D/A converter 150. Then the analog video signal is inputted to the encoder 160 to be encoded into a combined video signal, and a double screen video signal (DSS) is outputted.
The clock/timing generator 190 receives a clock signal generated by the first and second oscillators 170,180, and synchronous signals (Sync1,Sync2) for a main screen and a sub-screen generates clock signal/timing pulse signals required in the first and second A/D converters 100,110, the line memory 120, the frame memory 130, the mixing unit 140, the D/A converter 150 and the encoder 160. The first clock signal oscillator 170 receives the horizontal synchronous signal (Sync1) for a main screen to generate a clock signal corresponding to a phase locked loop(hereinafter, called PLL)(not illustrated) provided therein, as shown in FIG. 2C, and the second clock signal oscillator 180 receives the horizontal synchronous signal (Sync2) for a sub-screen to generate a clock signal corresponding thereto, as shown in FIG. 2D. Here, the PLL is also used.
However, a division ratio of the PLL is increased by using the PLL so as to generate a clock signal corresponding to the horizontal synchronous signal, resulting in generating a noise or a jitter in accordance with a non reference signal.
Moreover, when a reference signal and a non reference signal are inputted as video signals for a main screen and a sub-screen, the phenomenon the sub-screen is cut off into top and bottom portions occurs due to a jitter and a difference of a vertical frequency.
Further, when two screens are displayed on a television having a larger width, if the screen ratio is set to be 4:3, nothing is displayed at the upper and lower portions of the double screen pictures thereof on a 16:9 screen, and when a display is carried out on an entire 16:9 screen, the screen ration becomes 8:9, resulting in lengthening a screen.